Figure Captions

Fig. 1.
Sketch of the solar wind interaction with the ionosphere of
Venus [after Crawford et al., 1993].
The nightside ionosphere of Venus has
considerable structure. Regions known as "holes", containing enhanced radial
magnetic field and reduced plasma density, are often observed. These holes
appear to act as ducts for upgoing whistler-mode waves.

Fig. 2.
Example of plasma wave and magnetic field data for orbit 526.
These data are typical of the 100 Hz waves that have been attributed to lightning
generated whistler-mode waves. The upper panels show the wave intensity in the
four channels measured by the OEFD. The bottom four panels show the magnetic
field data cast into radial-east-north coordinates.

Fig. 3.
Example of wideband bursts observed on orbit 501. Similar in
format to Figure 2.

Fig. 4.
Fractional occurrence for 100 Hz only bursts. Data from the
first 22 seasons of nightside periapsis passes have been binned as a function of
radial distance and solar zenith angle.
The x-axis is along the Venus-Sun line, and
the Sun lies to the left of the figure. The vertical axis gives the distance
perpendicular to the Venus-Sun line,
=
(y
+ z).
The data have been binned
using 0.05
R
by 3° bins,
and then smoothed and interpolated to generate the plot.

Fig. 5.
Fractional occurrence rate for 5.4 kHz bursts. Similar in format
to Figure 4. Three different 5.4 kHz signals are observed, as indicated by the
labels "A", "B" and "C".

Fig. 6.
VLF/ELF Burst rate as a function of local time for the low
altitude bursts detected in the first three
seasons of nightside periapsis passes
[after Ho et al., 1991].

Fig. 8.
100 Hz burst rate dependence on the magnetic field orientation
[after Ho et al., 1992]. The upper plot shows
the time spent as a function of
(the angle between the magnetic field and the radius vector)
and
(the angle between the magnetic field and the spacecraft velocity vector).
Note that, for
0°,
90°.
The lower plot shows the burst rate, which peaks at
0°.
At higher
there is no dependence on
.

Fig. 9.
Whistler-mode dispersion curves for different propagation
angles [after Strangeway, 1992].
Wave frequencies are normalized to the electron
gyro-frequency, while the parallel wave vector is normalized to the inverse of the
plasma skin depth. The shaded regions show where thermal electron Landau and
gyro-damping are expected to occur.

Fig. 10.
The convective damping rate of 100 Hz whistler-mode waves
as a function of the normalized electron thermal velocity (=
).
The observed
magnetic field strength, electron density and temperature are used to calculate the
damping rate for parallel propagating whistler-mode waves at 100 Hz, assuming
an isotropic plasma. The thick line passing through the data gives the median
as a function of the damping rate, binned every half decade. A damping rate of
8.686 dB/km corresponds with 1 e-folding of the wave amplitude/km.

Fig. 11.
Scatter plot of burst occurrence as a function of electron
density and magnetic field strength. Various limiting curves are also shown. The
lower hybrid drift instability is most likely to occur for low
,
low
v/,
and low
k.
The whistler-mode requires low
only. At any particular density, lower
values of
,
v/,
and
k
lie above the limiting curves shown.

Fig. 12.
Diagram illustrating the hypothesis that lightning is a source for plasma
waves in the nightside ionosphere of Venus.